The present invention relates to semiconductor lasers, and more particularly to the structure of a semiconductor laser featuring a high modulation speed and a low threshold current.
The invention further relates to quantum well-type semiconductor lasers which oscillate on a small threshold current that was not realized so far, and particularly to semiconductor lasers adapted to opto-electric integrated circuits or to optical integrated circuits.
The modulation speed of a semiconductor laser is proportional to the maximum frequency of the modulation of the semiconductor laser. In order to raise the speed of a semiconductor laser, accordingly, the maximum frequency in the direct modulation of the semiconductor laser needs to be rendered as high as possible. Usually, the maximum frequencies in the direct modulation of semiconductor lasers are approximately 5 GHz. Recently, it has been theoretically predicted that the maximum frequency will rise with the so-called quantum well type laser in which the thickness of an active layer is smaller than an electron wave packet within a crystal (Y. ARAKAWA et al.: Applied Physics Letters, 45, 950 (1984)). On the other hand, it has been experimentally verified as to conventional semiconductor lasers that the maximum frequency rises when an active layer is heavily doped with an impurity (C. B. SU et al.: Applied Physics Letters, 46, 344 (1985)). In either case, however, the maximum frequency of the direct modulation is near 10 GHz without any other special contrivance.
In the Record of the 9th Laser Conference, pp. 162-3, it has been disclosed by C. B. SU et al. that the modulatible frequency of a semiconductor laser increases when the impurity concentration of a p-type active layer is raised. This is based on the fact that the increment of a gain coefficient versus the increase of injected carriers is enlarged by impurity doping. This method, however, has had the problem that the lifetimes of the carriers of the active layer shorten to raise the threshold current and to lower the radiation efficiency.
Furthermore, it has been urged to provide a semiconductor laser which oscillates on a small threshold current, i.e., to provide a semiconductor laser which consumes electric power in small amounts as a source of light for optoelectric integrated circuits (OEIC's) or for optical integrated circuits (OIC's). A method to decrease the threshold current by employing an active layer of the quantum well type and utilizing the quantum size effect, has been announced by Sugimoto et al., in Tsushingihoto, published by the Japanese Association of Electronic Communications, Vol. OQE 85-78, p. 85. According to this method, the threshold current is about 8 mA, which is about one-half the threshold current of 20 mA of the conventional semiconductor laser of the double heterostructure.
According to the above-mentioned conventional art, the device structure of the quantum well active layer has been almost optimized. With the conventional quantum well active layer, therefore, it is difficult to decrease the threshold current to be smaller than the above-mentioned value (about 8 mA). However, the threshold current of this level is too great for the source of light for OEIC's, and must further be decreased to realize OEIC's having many functions and in a highly integrated form.